High frequency electric-field-driven instability in the electron presheath.

Recent work has shown that the electron sheath near a probe biased above the plasma potential is accompanied by an electron presheath. The electron presheath extends several times further into the plasma than its ion rich counterpart and provides a large scale equilibrium electric field. Linear analysis of the Vlasov equation predicts that such an equilibrium electric field can excite an instability near the electron plasma frequency. Furthermore, using a BGK collision operator for electron-neutral collisions shows that the instability is excited over wavelengths on the order of 1-10 electron Debye lengths. Higher neutral pressure narrows this range, while higher electric field strength increases the maximum growth rate. Properties of this predicted electric field driven instability have been tested in a low-temperature laboratory plasma experiment, where the electron sheath is provided by a positively biased electrode immersed in an argon plasma. Measurements of wave correlations have been done in an attempt to distinguish the electric-field driven instability from a sheath-plasma resonance instability.